By combining two fundamentally different microscopy techniques, researchers can now measure the optical properties of a sample with pinpoint accuracy. The original goal was to investigate biological samples on a molecular scale—but this soon led to stubborn technical problems. Eventually, however, the researchers realized that the very source of their frustrating measurement inaccuracies—the variable refractive index of the sample—could itself be determined precisely. When two fundamentally different microscopy methods are combined, this former source of error turns into a highly informative measurement result.